The invention relates to mass spectrometers and methods of mass spectrometry.
A mass spectrometer is capable of ionising a neutral analyte molecule to form a charged parent ion that may then fragment to produce a range of smaller ions. The resulting ions are collected sequentially at progressively higher mass/charge (m/z) ratios to yield a so-called mass spectrum that can be used to “fingerprint” the original molecule as well as providing much other information. In general, mass spectrometers offer high sensitivity, low detection limits and a wide diversity of applications.
There are a number of conventional configurations of mass spectrometers including magnetic sector type, quadrupole type and time-of-flight type.
In a time-of-flight mass spectrometer the same kinetic energy is given to all ion species irrespective of mass-to-charge ratio. This is done by accelerating the ion packets in an electric field formed between an extraction grid electrode and an accelerator grid electrode. The amount of acceleration is dictated by the voltage difference between these two electrodes. For example, the accelerator electrode may be held at V=10 kV above the extraction grid electrode voltage. Another way of expressing the fact that all ion species are given the same kinetic energy is to say that the lighter, higher charge state ions are accelerated to a higher velocity and the heavier, lower charge state ions are accelerated to a lower velocity, i.e. the velocity is inversely proportional to mass-to-charge ratio, more precisely inversely proportional to the square root of mass-to-charge ratio m/z according to the equation:
      1    v    =            1                        2          ⁢                                          ⁢          V                      ⁢                  m        z            where v is velocity, V is the voltage between the extraction and accelerator electrodes, m is the mass of the ion species and z is its charge.
More recently, one of the present inventors has developed a new type of mass spectrometer that operates according to a different basic principle, as described in U.S. Pat. No. 7,247,847B2 [1], the full contents of which are incorporated herein by reference. The mass spectrometer of U.S. Pat. No. 7,247,847B2 accelerates all ion species to nominally equal velocities irrespective of their mass-to-charge ratios to provide a so-called constant velocity or iso-tach mass spectrometer.
To accelerate all ion species to nominally equal velocities irrespective of their mass-to-charge ratios, the mass spectrometer of U.S. Pat. No. 7,247,847B2 is provided with a specially designed mass filter in which the electrodes are driven with an exponential voltage pulse, as schematically illustrated in FIG. 1. A packet of ions entering the electrode region therefore experience a time dependent instantaneous voltage Vt which increases exponentially with time according to the formula Vt=V0 exp t/τ where V0 is the voltage at t=0 and r is the exponential time constant. This contrasts from a time-of-flight design in which the accelerating voltage V is constant, i.e. time invariant. U.S. Pat. No. 7,247,847B2 refers to the mass filter as providing an “exponential box” for accelerating ions of an ion packet to substantially equal velocities. The mass filter (sometimes referred to as an analyser) comprises an electrode arrangement and a drive circuit, the drive circuit being configured to apply the exponential voltage profile to the electrode arrangement.
FIG. 2 shows a schematic diagram of the drive circuit 100 disclosed in U.S. Pat. No. 7,247,847B2. The drive circuit comprises three main functional parts. These are a low voltage waveform generator 102, a wideband amplifier 104 and a step-up transformer 106. The low voltage waveform generator 102 and the wideband amplifier 104 are used to produce an exponential pulse shape and the step-up transformer 106 is necessary to achieve the high voltages used to drive the mass spectrometer electrodes.
Although the drive circuit disclosed in U.S. Pat. No. 7,247,847B2 functions as required, it is relatively complicated and costly to build. In particular, the requirement to produce an exponential voltage pulse necessitates that the amplification stages have high bandwidth, since the exponential voltage pulse has its power spread over a wide frequency range.